Memory devices are conventionally known which comprise a memory typified by a NAND flash memory and which execute a plurality of access commands in parallel. However, flash memories such as NAND flash memories are known to have a relatively low data retention capability. Thus, the quality of data stored in a flash memory may, for example, be degraded over time.
Hence, conventional memory devices with flash memories are appropriately checked for degraded quality of the data stored in the flash memory. The check involves read accesses and is carried out, for example, in units of blocks by a memory controller. If degraded quality is detected, the data in the corresponding block is rewritten to another block in the flash memory. Consequently, the data in the block detected to be degraded in quality is refreshed. This series of processes is called a patrol refresh process.
In the patrol refresh process, a plurality of access commands (read commands) are speculatively issued in order to gain efficient read access to the entire area of a block to be checked (patrolled). That is, succeeding access commands are issued before preceding access commands are completely executed. Thus, the memory controller is configured to record all the issued commands in order to deal with a possible error in read accesses carried out in accordance with the access commands. This is not limited to the patrol refresh process. Namely, the memory controller is configured to record all the access commands speculatively issued for increased operation speed in a read/write process intended for the entire area of the block.
Power to a memory device with a flash memory may be lost while the data in a block in the flash memory is being rewritten. In this case, the rewriting may fail to complete normally. Thus, memory devices are also known which are configured such that when the data in a block in the flash memory is to be rewritten, information indicative of the progress of the rewriting (rewrite progress information) is recorded. When the memory device is powered on again, the data in the block in which the rewrite failed to complete normally is recovered based on the information on the progress of the rewrite recorded at the time of the power shutdown.
As described above, the conventional memory device with a flash memory is configured to record all the access commands speculatively issued in the read/write process (memory access process) intended for the entire area of a block. Thus, the conventional memory device requires a memory area (that is a management area) in which all the access commands are recorded. This also applies to memory devices with memories other than the flash memory provided that the memory devices execute a plurality of speculatively issued access commands in parallel.